The present invention pertains to processes involving the deposition of one or more fluid materials onto a substrate.
There are many industrial processes where accurate or controlled deposition of fluid substances is desirable. While this is true for the printing industry, especially with respect to inkjet printing, it also applies to other manufacturing processes, particularly in the electronics and microelectronics industry. Examples include processes involved in the manufacture of display screens and other electronic components.
Recent advances in inkjet printing technologies have vastly improved inkjet printing as a means for the precise deposition of liquid droplets. Because inkjet printing makes it possible to deposit very small measures of fluid very accurately and cheaply, and because inkjet deposition typically requires fewer steps and less equipment than conventional methods such as photolithography, it is a very attractive candidate for many precision-manufacturing applications outside of conventional printing. Outside of the graphic arts, examples of proposed applications of inkjet printing include manufacture of color filters for flat panel displays, manufacture of OLED devices, deposition of microlenses on the tips of optical fibers, deposition of conductive ink in the manufacture of circuit boards, and the manufacture of 3-dimensional objects.
In many of the proposed applications, the precise placement of the fluid substance, whether ink or polymer or other material, is critical. In the case of color filter manufacture for conventional flat panel displays or OLED devices, many such precisely placed fluid droplets must be placed very close to other such droplets without coalescing. This is particularly important in situations in which different kinds of fluids are being deposited on the same substrate, such as differently colored inks on a color filter substrate. In the case of color filters, it is vital to the quality of the filter to prevent blending between discrete ink droplets, as differently colored filter elements are typically situated adjacent to one another on a clear substrate, separated only by a minute barrier on the order of some microns across. In the case of organic light emitting diodes (OLED) and semiconducting polymers, the actual active device materials also need to be deposited with great accuracy and circumspection in order to ensure that the various sections of the devices remain well delineated.
The use of fluid droplet deposition techniques, for precision manufacturing applications does, however, present a number of difficulties. Firstly, inkjet technology typically has a spatial accuracy of approximately 40 microns while the display screen and semiconductor industries require accuracies at least an order of magnitude better. Secondly, there are particular problems with respect to controlling the position of the fluids once they have been deposited upon a substrate.
For example, in the case where the fluid material is intended to adhere to the substrate, problems arise because the ability of the fluid to adhere to a surface is related to its ability to wet a surface, and the fact that a fluid capable of wetting a surface will also tend to spread upon that same surface. This can cause undesirable effects such as coalescence of adjacent droplets as well as blending of the fluids in adjacent droplets. Droplet spreading can also lead a non-uniform thickness of the layer on the substrate, as the fluid droplet layer may tend to be thinner at its boundaries when it spreads.
In applications where it is necessary to deposit a pattern of one or more fluid substances that need to be contained within specific areas, the wetting of the substrate will prevent accurate and uniform placement of droplets, as well as non-uniform thicknesses in layers created with the fluid droplets. At the same time, the droplets are generally required to adhere well to the substrate upon which they are deposited.
One area in which the specific aforementioned problem can be an issue is the manufacture of color filters for flat panel displays. Manufacturers are faced with a stringent requirement for the accurate placement of fluid substances without being able to tolerate any coalescence or blending between fluid droplets from adjacent patterned cells. Filter layer thickness and the distribution of colorant in the layer are also important to ensure that each colored region transmits light uniformly.
The color filter of a liquid crystal display is typically fabricated with alternating cells of red, green, and blue material deposited on a transparent substrate at the same matrix spacing as the panel. The colored materials act as filters and therefore transmit light. To enhance contrast, a light-shielding region, known as a black matrix, usually demarcates the color cells. This black matrix typically has an optical density of greater than 3.0 in order to adequately block stray light from adjacent cells. It is also used to mask non-emitting areas such as LCD cell electrodes.
Many different methods have been used or proposed for forming the partition walls. Typically, photolithography is used. However, other methods such as electrodeposition, screen offset printing, gravure printing, flexographic printing, and inkjet printing have also been suggested in the art.
Most of these methods suffer from a number of deficiencies. In general the very precise methods are expensive and complicated, and often require additional processing steps or expensive equipment. The printing based methods tend to be cheaper, but tend to suffer the fact that the partitions are being made by applying a liquid to a surface, with the same attendant problems of liquid spreading and coalescence. Conventional manufacturing techniques like photolithography are costly due to the need for complex processes like vacuum chamber deposition as well as the multiple steps which are required to produce a finished color filter. On the other hand, currently available alternative methods are either just as complex, or lack the required precision. The resulting high fabrication costs have thus prevented flat panel displays from replacing conventional CRT displays in applications where cost is a primary concern. This has happened despite their many desirable characteristics, such as compact size and low radiation emissions.
Other types of display devices, based on OLEDs, have requirements slightly different from conventional color filters. For example, because OLEDs are light emitting, the optical density required for the partition walls may not be as severe and hence controlling the thickness of the fluid layers may be less important. However, accurate placement is still needed. For example, an OLED display may be made up of OLED cells of different colours, which again would require that closely placed fluid droplets be prevented from mixing or coalescing.
A number of methods of addressing this problem have been proposed, and the concept of using the surface energy characteristics of the fluids, substrates, and the partition barriers to control the placement of inkjetted droplets has specifically been suggested. Again, as with previously mentioned approaches to fabricating partition walls in general, the various approaches proposed for producing a desirable pattern of surface energy variations on a substrate of a colour filter have tended to be either expensive or complex. Additionally, they frequently require many processing steps, including post-processing steps, or lack the precision required for the manufacture of devices such as color filters for flat panel displays or OLED displays.
The incorporation of conventional printing techniques has been proposed in various approaches, since the conventional printing industry has extensive experience with methods to produce large numbers of precisely patterned objects at low cost. However, the use of conventional printing techniques requiring the use of liquid ink to produce precisely patterned surface energy variations on a substrate suffers the same problems mentioned earlier with regard to adhesion, wettability, and liquid spreading.
One well-known printing technique that does not involve the deposition of a fluid in order to create a precise pattern on a surface is thermal transfer. This technology is employed extensively in the printing and imaging field for the purposes of proofing. In applying this technology to the filter, OLED and polymeric semiconductor devices addressed here, it has the benefit of not employing liquids that could spread or coalesce.
In thermal transfer, a donor material layer is disposed on a carrier sheet. This medium is then either placed in close proximity with a substrate or in contact with that substrate, such that the donor layer faces the substrate. The donor layer is then transferred from the carrier sheet to the required areas of the substrate by a variety of means. These include illumination with a laser or another thermal source. When a laser is employed in this fashion, the process is referred to as laser induced film transfer.
The mechanism of transfer varies from essentially explosive heating of the interlayer between the donor and carrier to detach a minute amount of donor material, to phase change processes. When a phase change is employed, the process is known as phase change transfer. All the mechanisms have the net result of transferring donor material in a precisely defined fashion from the carrier sheet to the substrate on which the image or pattern is required. Provided a well-defined laser-beam is used, images or patterns may be imagewise written onto the carrier to transfer the donor material imagewise to the substrate. This technology provides a method to obtain very well-defined edge structures and is inherently capable of the precision required for the devices discussed here.
Unfortunately, there has to date been no simple method proposed for incorporating thermal transfer into an inkjet-based method for manufacturing colour filters, OLED devices or semiconducting polymer devices.
A method for forming a partition layer on a substrate by imaging a thermal or laser transfer medium onto the substrate using a imagewise controlled radiation source is described. A matrix of partition cells is created on the substrate that can, in a further step, be selectively filled with fluid. The selective filling of partition cells can be accomplished using an inkjet printing technique. The partition layer, the substrate, and fluids to be deposited are selected so that the fluids wet the substrate but not the partitions, preventing unwanted interactions between fluids in adjacent cells. An apparatus for implementing this method is also described.